1. Field of the Invention
The present invention relates to an image heating device, which is suitable for mounting in an image forming apparatus such as a copying machine or a printer using a recording technique of an electrophotographic or an electrostatic recording system, and, more particularly, to an image heating device having a function of shutting off heat generation in case where temperature excessively rises.
2. Related Background Art
Heretofore, as an image heating device represented by a heat-fixing device, there has been widely used a fixing device of a thermal roller type.
The thermal roller type fixing device has a press-contacting roller pair consisting of a fixing roller (heating roller) and a pressure roller as the fundamental structure, and the pair of rollers are rotated to allow a recording material having an unfixed image formed thereon to be introduced to a fixing (heating) nip portion, which is a mutually press-contacting portion of the pair of rollers, so as to be sandwiched and conveyed, so that the unfixed image is thermally pressure-fixed on the recording material by the heat of the fixing roller and the pressure force of the fixing nip portion.
The fixing roller generally has a hollow metal roller of aluminum as a substrate (core bar), and in the internal space thereof is provided a halogen lamp as a heat source, and the energization to the halogen lamp is controlled so as to heat the fixing roller by heat generation of the halogen lamp and maintain an outer periphery thereof at a predetermined fixing temperature, so that the temperature is adjusted.
On the other hand, in Japanese Patent Application Laid-Open No. 7-114276 and Japanese Patent Application Laid-Open No. 11-143272 is disclosed an induction heat-fixing device, which induces an electric current in a fixing film by a magnetic flux and a heat is generated therein by the joule heat. This makes it possible to allow a rotating member for fixation such as a fixing film and a fixing roller to directly generate a heat by using generation of an induction current, and makes it possible to achieve a fixing process of a much higher efficiency.
Incidentally, the above-described fixing device has usually a safety device or the like with a temperature detecting member such as a thermoswitch or the like.
This is, for example, to prevent an excessive temperature rise during stopping of rotation due to breakage of a rotational driving gear or run away driving due to malfunction of temperature control, and to detect the excessive temperature rise so as to stop heating operation of the device and secure the safety of the device.
Further, as an arrangement method of the temperature detecting member as the safety device, a method of such a constitution is employed, in which the temperature detecting member is held so as not to be in contact with the outer peripheral surface of the fixing roller or the film in order to prevent contact traces of the temperature detecting member from generating on the surface of the rotating member for fixation to develop image defects.
However, particularly in the fixing device employing a flexible rotating member for fixation (fixing film), depending on a supporting method of the temperature detecting member, there was the case where the following malfunction occurred. That is, with regard to the fixing device of a film type, since the heat conduction by the film itself is not high, such a constitution is desirable, wherein the arrangement position of the temperature detecting member is as close as possible to a heat generation area, that is, an area in which an eddy current is generated. However, for example, in the case where the constitution is such that the support member of the temperature detecting member is arranged in the heat generation area in the longitudinal direction of the fixing device so that the temperature detecting member is positioned close to the heat generation area, depending on the material of the support member and the input electric power, the support member may be affected by a leaked magnetic flux in the vicinity of the heat generation area to generate a heat by itself to effect temperature rise, and the distance between the temperature detecting member and the film is varied by thermal expansion, and at the worst, there has been the case where the temperature detecting member and the film were brought into contact with each other. In such a case, it is considered that the temperature detection may be not properly performed as a safety device, and heating operation may be stopped even within the permissible temperature range, and the contact traces may be developed on the image by flaws generated by contact between the temperature detecting member and the outer peripheral surface of the film.
Although it is certainly possible to set a large distance between the temperature detecting member and the outer peripheral surface of the film, if the distance is too large, malfunctions may occur such as the temperature detection being not correctly performed as the safety device and the like even during excessive temperature rise, and there has been a limit imposed on the distance.
The problems such as described above tend to occur more frequently in the fixing device having a flexible rotating member for fixation (fixing film) than in the fixing device having a rigid rotating member for fixation (fixing roller) because the magnetic flux which leaks outside of the rotating member is greater in the former than in the latter.